Computational Investigation on Pentagonal Monolayer HgO2 with Ultrahigh Anisotropic Carrier Mobility and Low Gibbs Free Energy for Photocatalytic Water Splitting

Searching for photocatalysts that can make full use of solar light to catalyze the decomposition of water into hydrogen and oxygen is of great significance for the development of renewable energy. Through systematic structure search and density functional theory computations, a two-dimensional penta-HgO2 monolayer as a candidate with great potential of photocatalysts for hydrogen evolution reaction is uncovered. The stability of the penta-HgO2 monolayer is confirmed through energetic analysis, phonon spectra, ab initio molecular dynamics simulations, and mechanical property evaluations. The penta-HgO2 monolayer is a semiconductor, which has a medium indirect band gap of 2.93 eV, and its band edge positions perfectly satisfy both the oxidation and reduction potentials of water. Furthermore, the penta-HgO2 monolayer exhibits rather pronounced light absorption (∼105 cm–1) in both visible and ultraviolet regions. Along the x/y transport direction, the penta-HgO2 monolayer has fascinating anisotropic carrier mobility for electrons (4115.63/736.80 cm2 V–1 S–1) and holes (330.29/102.01 cm2 V–1 s–1). More importantly, the penta-HgO2 monolayer shows high catalytic activity with a low Gibbs free energy for hydrogen evolution reaction, and biaxial tensile strain can significantly improve this property. The suitable band alignments, ultrahigh anisotropic carrier mobility, excellent optical adsorption properties, and high catalytic activity render the penta-HgO2 monolayer to be a hopeful candidate for high-performance photocatalytic water-splitting applications.